(1) Field of the Invention
This invention relates to a refrigerator with a manually-initiated defrost system for a separate freezer compartment, and particularly to a freezer defrost sensing system to assist the user in determining when the frost has melted sufficiently on the exterior of the freezer liner to warrant ending the defrost cycle.
(2) Description of the Prior Art
In a two-door refrigerator, there is a fresh food compartment and a separate freezer compartment. The fresh food compartment is cooled by evaporator coils located within the top portion of that compartment, while the freezer compartment is cooled by evaporator coils around the outside thereof. There are models of refrigerators where the fresh food compartment is self-defrosting, while the freezer compartment must be manually defrosted. Defrosting of the fresh food compartment takes place automatically every time the compressor turns off. A manually settable temperature control is located within the fresh food compartment and it causes the compressor to turn ON and OFF in order to maintain a preselected temperature. Frost collects on both the refrigerator coils and on the freezer compartment surfaces while the compressor is operating. When the compressor is OFF, the coils in the fresh food compartment are warm enough to defrost; however, the freezer compartment coils remain below freezing temperature and they do not defrost. Therefore, frost builds up on both the interior and exterior surfaces of the freezer compartment liner and it is necessary for the user to periodically turn the compressor off for a long period of time, for example a minimum of two hours, to insure complete defrosting of the freezer compartment. This should be done whenever frost becomes of a thickness between 1/4 to 1/2 inch thick in any area on the interior surfaces of the compartment. It is not wise to operate the refrigerator longer than six months without completely defrosting the freezer compartment.
There is a natural tendency for users to terminate a defrost cycle when all visible ice on the inner surfaces of the freezer compartment has melted. This would be contrary to the instructions of the appliance manufacturer for the proper length of the defrost cycle prescribed in the User's instruction manual, because the exterior surfaces of the freezer defrost last.
If the defrost cycle is terminated prematurely, frost would remain on the evaporator coils and the exterior surface of the freezer compartment that are hidden from sight, since a thick blanket of thermal insulation surrounds the freezer compartment and the insulation is located within the outer case of the refrigerator. During a freezer defrost cycle, the frost on the exterior surfaces will melt and water will run down through the low density fiber glass insulation. The lower portion of the insulation may be wet throughout. If the compressor is restarted prematurely, then this water in the insulation may freeze and cause a heavy ice buildup in the vicinity of the lower portion of the freezer compartment.
Attempts have been made in the past for assisting the user in knowing when a defrost cycle should be initiated to eliminate the frost buildup. U.S. Pat. No. 2,071,148 of Weisberg et al discloses a small freezer compartment or evaporator located within the fresh food compartment of a refrigerator. An indicator light is provided to be operated by a switch positioned on the exterior of the evaporator, that is in series with a refrigerator door switch. This first switch will close when a predetermined heavy layer of ice forms on the exterior of the evaporator. Actually, the layer of ice closes the circuit of the switch, thereby energizing the light when the refrigerator door is opened. In this Weisberg et al patent, the evaporator is visible to the user when the refrigerator door is open because the evaporator is located within the fresh food compartment, as in standard refrigerators that have been made for decades.
Later, single-door refrigerators with full width freezer evaporators were made available, and they had a separate evaporator door within the fresh food compartment. It then became difficult for the user to determine the amount of frost collected on the evaporator and especially at the back of the evaporator which is hidden from the view of the user. U.S. Pat. No. 2,644,422 of Carbary, which is assigned to the present assignee, discloses a mechanical indicating means which measures the thickness of the frost on the exterior rear surface of the evaporator whenever the evaporator door is opened, which would indicate to the user the thickness of the frost and the time when the freezer evaporator should be defrosted.
U.S. Pat. No. 3,065,608 of Arzberger discloses an automatically operable defrosting control system for controlling the refrigerator evaporator, and a separate automatically operable defrosting control system for the freezer evaporator. In the latter system, a temperature sensing bulb is fixedly mounted to the freezer evaporator coils, and it is connected by means of a capillary to a bellows that controls the movable contact of a switch in the compressor circuit. When the temperature of the freezer evaporator coils drops to a predetermined low level, indicating the existence of a coating of frost on the freezer evaporator coils, a bellows contracts to open the switch and interrupt the circuit through the compressor motor.
U.S. Pat. No. 3,839,878 of Tilmanis discloses a two-door combination refrigerator-freezer that has temperature sensing devices to monitor the temperature of the freezer evporator coils and the freezer compartment interior. The temperature sensing devices are thermistors mounted to the evaporator coil and one mounted within the freezer compartment. When the difference in these temperatures at those locations exceeds a predetermined value and one of the monitored temperatures is less than a predetermined value, the defrost operation will be initiated.
The principle object of the present invention is to provide a frost sensing means within the insulation surrounding a freezer compartment so as to be thermally sensitive to the temperature of the insulation so that when a preselected high temperature is reached it will be known that the defrost cycle may be terminated.
A further object of the present invention is to provide a frost sensing means of the class described to be located remote from the freezer evaporator so as to be able to determine when the frost on the exterior surface of this evaporator has melted.
A further object of the present invention is to provide a frost sensing means of the class described in combination with an indicating means, which indicating means when energized signifies that the frost level has become excessive and that a defrost cycle should be initiated, and when the indicating means is de-energized to signify that the defrost cycle may be terminated.
A further object of the present invention is to provide a modification with a frost sensing means of the class described with a manual means for initiating the defrost cycle and with thermally sensitive means for terminating the defrost cycle.